This invention relates to quantum well device manufacturing by vapor phase epitaxy, and in particular to the manufacture of strained quantum well devices.
For the purpose of this specification a quantum well stack is defined to mean, in respect of a single quantum well structure device, a single quantum well layer sandwiched between two barrier layers of higher bandgap material, and in respect of a multi-quantum well structure device, a plurality of n quantum well layers sandwiched alternately between (n+1) barrier layers of higher band gap material.
An example of such strained quantum well stack devices are the lasers described by M. Yamamoto et al., `Strained-layer InGaAs/InGaAsP MQW structures and their application to 1.67 .mu.m lasers grown by metalorganic vapor phase epitaxy`, Journal of Crystal Growth, Vol. 107, No. 1/4, January 1991 pages 796-801.
In a vapor phase epitaxy reactor system the reagents fed to a reaction chamber are derived from source lines, in each of which an individual reagent, or reagent mixture, flows with a flux that may be controlled independently of the fluxes in the other source lines of the system. Usually, but not necessarily, for any particular growth there is one source line feeding the reaction chamber per elemental component of that growth. The elemental composition of the material grown in the reaction chamber depends upon the fluxes of the reagents in the various source lines, and also upon other parameters such as temperature and pressure. When making the transition from growing one material to growing a different material it is not normally possible to change the flux in an individual source line, or the fluxes in a set of such lines, sufficiently quickly in order to be able to proceed with the growth without interruption. Such interruption is generally undesirable because of the risk of degradation of the exposed surface liable to occur in the interval before growth recommences. This problem is conveniently circumvented by employing different source lines for supplying the same reagent at different fluxes, and switching between these source lines in order to effect the desired changes in composition of the material being grown.
In any particular vapor phase epitaxy reactor system the settings of source line fluxes required to produce a desired composition (or bandgap and bulk lattice parameter) of growth are normally determined by an iterative procedure that involves growing a calibration layer with approximately appropriate flux settings, determining the composition (or bandgap and bulk lattice parameter) of the material actually grown, and then making appropriate adjustments to the fluxes before repeating the procedure. The determination of the composition (or bandgap and bulk lattice parameter) grown under any particular set of flux settings typically requires the growth of a calibration layer of a thickness that is greater than that for instance of a quantum well layer. Particularly in the case of attempting to grow strained quantum well layers, this can present difficulties because the bulk lattice parameter mismatch may be so great that the growth of a calibration layer of sufficient thickness is not possible without introducing such a high dislocation density in the grown material as to render it impossible, or at least very difficult, to make the bandgap and bulk lattice parameter measurements necessary to determine its composition.